1. Field of the Invention (Technical Field):
The present invention relates to engines, specifically to an engine utilizing an improved method for using external heat to heat a unit mass of working fluid and thereby convert the thermal energy to mechanical energy, where the unit mass is later expelled and a new unit mass of working fluid is introduced to repeat the cycle.
2. Background Art
The conversion of chemical and thermal energy to useful mechanical and electrical energy has been studied for hundreds of years. This interest has led to some engines widely used today that accomplish this feat, well-known examples being the internal combustion engines, and gas combustion- and steam-driven turbines. Unfortunately, all technologies currently in widespread use are limited in efficiency to approximately less than 40% and are constrained in the type of fuel that can be used.
One group of engines for converting energy known variously as heat engines, caloric engines, hot air engines or external combustion engines, have seen very little application. Exemplary engines in this field are the Carnot, Stirling and Ericsson engines. While such engines in theory are capable of remarkably high efficiencies, in practice the engines have failed to reach their full potential within a reasonable cost and package.
There are several reasons why Carnot, Stirling and Ericsson cycle engines have not been proven effective or broadly commercialized. Most important is the difficulty in achieving the heat transfer required during the isothermal heat transfer processes to reach a reasonable power output within a reasonable cost and package.
Because the Stirling and Ericsson engines are closed cycles that are typically under significant pressure, problems with design and sealing abound in containing the working fluid during operation. The stringent sealing requirements of these engines tend to increase mechanical friction.
The effectiveness of the regenerator or “recuperator” used in these engines is limited. There are some indications that they save 75% of the heat during the cooling constant volume process, and return it during the constant volume heating process. Nonetheless, an effectiveness of 75% results in a significant loss of thermal energy and efficiency.
The rate of heat transfer during the isothermal heat transfer process primarily is governed by the temperature difference between the working fluid and the heat exchanger. In order to maintain sufficient heat transfer rates to accomplish a reasonable power output, it is required to have rather large temperature differences. However, increasing the temperature differences effectively causes the working fluid hot temperature to drop and the cold temperature to increase, thereby decreasing efficiency.
Moreover, the critical components in Ericsson and Stirling engines, such as valves, cylinders and pistons, are subject to extremely high temperatures. While high temperatures are regularly seen in automotive engines and turbines, the Stirling and Ericsson engines are also required to maintain extreme temperature gradients to function properly. These extreme temperature gradients as well as high temperatures require that the engine be built primarily with exotic materials.
Because exhaust or waste heat in Ericsson and Stirling engines is typically rejected through the heat exchanger during the cold isothermal heat transfer process, the cooling capabilities required to maintain the heat exchanger temperature are prohibitive. In contrast, an internal combustion engine rejects at least 50% of waste heat through the hot exhaust gases.
The mechanical configurations of Stirling engines are generally divided into three groups. They are typically called Alpha, Beta and Gamma engines, thoroughly discussed in the website www.ent.ohiou.edu/˜urieli/stirling/engines/engines.html. In each of those Stirling designs, the hot exchanger, the regenerator and the cold exchanger are placed in series and in close proximity. The difficulties in thermally isolating each exchanger and preventing the heat from the hot exchanger from being transferred to the other two, and thus wasted, are well known. Additionally because they use three heat exchangers (hot, cold and regenerator), Stirling engines have excessive dead space that reduces specific power and efficiency.